The preset invention relates to a composite plating method of forming a composite film composed of fine particles and metal. More specifically, the present invention relates to a new method of forming a composite plating film in which the content of the fine particles can be controlled.
A composite plating method is conventionally known as a plating method in which fine particles of alumina, silicon carbide or the like are dispersed in a metal plating bath so that the fine particles are present in the eutectoid state in the plating metal.
The main effects achieved by a composite plating film obtained by such a method include (1) improvement of wear resistance, (2) improvement of lubricity, (3) improvement of corrosion resistance, (4) change in surface appearance, (5) improvement of mechanical properties of the plating, and the like. In order to achieve such effects in actual applications, it to desirable that the content of the fine particles in the metal is raised as high as possible.
In the conventional composite plating method as described above, a surfactant is added in order to disperse the fine particles or change the surface potential and then the mixture is stirred to effect electroplating. However, addition of a surfactant has only a limited effect of enhancing the content of the fine particles in the plating metal, although such addition of a surfactant can increase the content of the fine particles to some extent. It is assumed that the effect by a surfactant is limited because the surfactant remains as it is on the fine particles which have been deposited by plating in the adsorbed state and prevents other fine particles from being deposited.
Accordingly, such a problem associated with addition of a surfactant i.e., the difficulty of enhancing the content of the fine particles to a significantly high level which exceeds a certain content value has remained unsolved in the conventional technique.
An object of the present invention is to solve the aforementioned problems and provide a method of forming a composite plating film in which the content of fine particles can be increased. Specifically, the present invention provides a composite plating method, characterized in that it comprises the steps of: adding inorganic or organic fine particles which are insoluble to water, to a metal plating bath, by dispersing the fine particles in a watery medium by the help or an azo-surfactant having an aromatic azo compound residue; and effecting electrolysis, thereby forming a composite plating metal film composed of the fine particles and a metal.
Further, the present invention provides a composite plating metal film formed by the aforementioned method.